The underlying working hypothesis of this research program was that destruction of tumor cells could be induced by a cell-mediated immunologic response directed not against tumor-specific antigens (TSA), but against (I) the antigenic determinant(s) of (enhancing) antibodies which combine with the tumor-specific antigens, (II) the distinct antigenic determinants of tissue components located primarily, if not exclusively, within the tumor foci, i.e. against the unique determinants of the fibrin matrix formed within some solid tumor foci. As an extension of last year's observation that cancer destroying cytotoxic cells could be derived in vitro from the culture of a methylcholanthrene-induced sarcoma in Sewall Wright strain 13 guinea pigs, experiments in vitro with 9 lines of human tumors have been recently initiated. As an offshoot of the current year's investigations, it has been shown that "suppressor" cells capable of inactivating "killer lymphocytes" exist in tumor-bearing hosts; this observation is being actively followed up since it may have far reaching implications, i.e. these cells may constitute yet another type of "blocking" factor(s). The biological alkylating agent Trenimon (2,3,5-trisethylenimino-1, 4-benzoquinone) binds readily to partially reduced proteins including immunoglobulins. The complex retains antibody activity and is an active alkylating agent. Immunologically specific cytotoxicity was demonstrated in vitro with Trenimon conjugated to rabbit IgG containing antibodies to a methylcholanthrene-induced sarcoma of strain 13 guinea pigs. These results indicate that it may be possible to use complexes of Trenimon with antibodies as a "homing" device for carrying the drug selectively to various target cells.